Abstract

Recently, it has been demonstrated that by recovering the amplitude and phase of the backscattered optical signal, a Phi OTDR using pulse coding can be treated as a fully linear system in terms of trace coding/decoding, thus allowing for the use of tens of thousands of bits with a dramatic improvement of the system performance. In this communication, as a continuation of previous work by the same authors, a preliminary study aiming at characterizing the limits of the system in terms of maximum usable code length is presented. Using a code exceeding 1 million bits over a duration of 0.26ms, it is observed that fiber optical path variations exceeding approximate to pi occurring over a time inferior to the pulse code length can lead to localized fading in the Phi OTDR trace. The occurrence, positions and form of the fading points along the Phi OTDR trace is observed to be strongly dependent on the type, frequency and amplitude of the perturbations applied to the fiber.